Quick stuff - the Justice Department is alleging that T-Mobile and ATT are competitors in the wireless business:

AT&T, with approximately 98.6 million connections to mobile wireless devices, and T-Mobile, with approximately 33.6 million connections, serve customers throughout the United States, with networks that each reach the homes of at least 90 percent of the U.S. population. AT&T and T-Mobile are two of only four mobile wireless providers with nationwide networks and a variety of competitive attributes associated with that national scale and presence. The other two nationwide networks are operated by Verizon Wireless ("Verizon") and Sprint Nextel Corp.

Sounds obvious, but defining competition and competitors is at the heart of the formula for merger/acquisition approval. Thus, the formula here is a 4 to 3 (from 4 competitors to 3.) This is not per se, an unapprovable acquisition because of that. The Government has to prove more (though the issue of the FCC's discretion is one I need to study and understand. I am providing a purely antitrust law as I understand it and experienced it analysis.) More as a I read.

Why taking T-Mobile would have an anti-competitive effect beyond the 4 to 3:

T-Mobile in particular -a company with a self-described "challenger brand," that historically has been a value provider, and that even within the past few months had been developing and deploying "disruptive pricing" plans -places important competitive pressure on its three larger rivals, particularly in terms of pricing, a critically important aspect of competition. AT&T's elimination of T-Mobile as an independent, lowpriced rival would remove a significant competitive force from the market.

According to Justice, T-Mobile is a price competition driver in the market. More as a I read.

Defining the relevant market:

Mobile wireless telecommunications services include both voice and data services (e.g., texting and Internet access) provided over a radio network and allow customers to maintain their telephone calls or data sessions wirelessly when traveling. Mobile wireless telecommunications providers offer their services on a variety of devices including mobile phones, smartphones, data cards, tablet computers, and netbooks. In addition, an increasingly important group of customers are building mobile wireless capability into new devices, such as e-readers and vehicle tracking equipment, and contracting for mobile wireless telecommunications services on behalf oftheir own customers. There are no cost-effective alternatives to mobile wireless telecommunications services. Because neither fixed wireless services nor wireline services are mobile, they are not regarded by consumers of mobile wireless telecommunications services as reasonable substitutes. In the face of a small but significant price increase imposed by a hypothetical monopolist it is unlikely that a sufficient number of customers would switch some or all of their usage from mobile wireless telecommunications services to fixed wireless or wireline services such that the price increase or reduction in innovation would be unprofitable. Mobile wireless telecommunications services accordingly is a relevant product market under Section 7 of the Clayton Act, 15 U.S.c. § 18.

How the relevant geographic markets are determined by Justice:

Mobile wireless telecommunications services are sold to consumers in local markets that are affected by nationwide competition among the dominant service providers. It is therefore appropriate both to identify local markets in which consumers purchase mobile wireless telecommunications services and to identify the nature ofthe nationwide competition affecting those markets. AT&T's acquisition ofT-Mobile will have nationwide competitive effects across local markets.

[. . .] The nation's four largest providers of mobile wireless telecommunications services, including AT&T and T-Mobile, provide and market service on a nationwide basis. Other providers have limited networks that cover only particular localities and regions. Those smaller carriers typically do not market to customers outside of their respective network coverage areas, and may not even sell to such customers; therefore, local or regional carriers are not an attractive, or perhaps even available, option for those customers who live and work in areas outside of these smaller providers' respective network coverage areas.

Accordingly, from a consumer's perspective, local areas may be considered relevant geographic markets for mobile wireless telecommunications services. The Cellular Market Areas ("CMAs") that the FCC has identified and used to license mobile wireless telecommunications services providers for certain spectrum bands often approximate the areas within which customers have the same competitive choices. AT&T and -Mobile compete against each other in local markets across the United States that collectively encompass a large majority of U.S. mobile wireless telecommunications consumers